1. Field of the Invention
The present invention generally relates to momentum transfer systems and, more specifically, to an improved fluid actuating system for imparting movement to a body, such as a spacecraft, in three orthogonal planes.
2. Description of the Prior Art
In moving vehicles which must accurately move through three orthogonal planes, artisans have developed many types of actuating systems which initiate movement in those three planes. In the design of spacecraft, a very unique set of problems has been presented to past artisans in their design of actuating systems. First, the operating environment is one of an absence of gravity. This restricts the system design alternatives. Also, a spacecraft steering/actuating system requires a high degree of accuracy and reliability. Once a spacecraft is launched and remains in space, the actuating system must have very low maintenance requirements, since any significant maintenance or repair may not then be possible. With the ever-increasing costs associated with the development, manufacture, and launch of any spacecraft, a maximization of the potential life span of the spacecraft is particularly important. Additionally, while spacecraft instrumentation has become more compact, the amount of instruments as well as equipment, such as for testing, has increased. As a result, minimization of wasted space, or maximizing the use of available space, is always a major concern to designers.
A typical design in the past to give spacecraft pitch, yaw, and roll control has been the use of retro rockets or thrusters placed on the periphery of the exterior of the spacecraft. Retro rockets or thrusters necessarily expel a propellant mass which is carried on board as part of the original payload. In conventional systems, the propellant is not retrievable. Therefore, the life of the spacecraft is necessarily limited to the supply of propellant which was part of the original payload. This, of course, also means that a longer life span of the spacecraft requires a larger amount of propellant which will take up space that might otherwise be available to instrumentation or testing equipment. Apart from the mass and volume considerations, rockets and thrusters present undesirable force profiles to the spacecraft during maneuvering. The expulsion of a propellant produces a difficult-to-modulate force that tends to ring the spacecraft as a function of the spacecraft's natural frequency and damping. Such ringing or resonance tends to degrade the performance of instrumentation mounted in the spacecraft. Further, retro rockets or thrusters are difficult to utilize for precision pointing because of the nature of their sudden, initial expulsion of propellant.
Reaction wheels have also been utilized in the past to orient spacecraft. While reaction wheels may solve some of the problems associated with thrusters, they too have drawbacks. Their construction tends to limit their range of utility. Conventional reaction wheel systems appear suitable for momentum storage capabilities up to about 1000 Nms Beyond that, their motors (which provide a torque source) and their reaction masses need considerable support structure, which not only adds to the overall payload that must be carried by the spacecraft, but also takes up available room that might otherwise be used for instrumentation or other devices. Also, large, robust bearings must be selected for the support of very large reaction wheels in order to prevent damage occurring to the bearings where launch environment accelerations are felt by the reaction wheels. If the bearings are not adequately sized, the bearings will sustain microscopic deformations that degrade their ability to turn smoothly. In addition, larger bearings are inherently less smooth than smaller bearings of equivalent precision. Because the fluid reaction system only needs bearings for its motor and pump (if the pump is mechanical), the bearings can advantageously be quite small.
Another attempt to overcome problems associated with retro rockets is disclosed in U.S. Pat. No. 4,662,178. Therein, a rotator apparatus is provided within the spacecraft and includes a channel arranged in a stacked array of loops. The channel contains a flowable material, and the channel is moved between first and second configurations. In the first configuration, the fluid is accelerated through the channel and is then maintained at a constant velocity until the spacecraft has achieved a desired attitudinal change. At that time, the channel is moved into the second configuration wherein the fluid is decelerated. The entire rotator apparatus is attached to the spacecraft by a rotatable shaft such that rotation which the rotator apparatus produces is transmitted to the main structure of the spacecraft as a counterrotation. A major problem with this design is the large amount of area within the spacecraft and through which the rotator apparatus sweeps in operation. This takes up precious space needed for other vital equipment. The present invention does not require this volume, as it need not change configuration.
Spacecraft design technology, as well as other related technologies, still requires an improved actuating system for attitude control that minimizes the space requirements within the spacecraft, provides precise control of the spacecraft, and minimizes the need for maintenance and repair by utilizing a simple design.